SPIE Optical Systems Design: reviewing Kingslake and polishing up ELT

11 Apr 2024

Highlights from Tuesday’s meetings co-located with SPIE Photonics Europe and exhibitions.

by Matthew Peach in Strasbourg

Tuesday’s twin conferences SPIE Photonics Europe and SPIE Optical Systems Design and the associated expo of more than 100 exhibiting companies and industry groups picked up where Monday’s sessions had left off.

Tuesday highlights

The Optical Systems Design plenary session opened with Prof. Julie L. Bentley from the Institute of Optics, at the University of Rochester, NY, United States, who reviewed the legacy and impact of Rudolf Kingslake, an English academic, lens designer, and engineer (1903-2003). In 1929, Kingslake was invited to teach at the University of Rochester, where he founded the Institute of Applied Optics, now known as The Institute of Optics.

Bentley’s talk was entitled “Lessons in lens design from Rudolf Kingslake: in the modern computing era can we learn anything new from the past?” She said, “Rudolf Kingslake is widely regarded as one of the founders of modern optical design. When educating his students at The Institute of Optics, he championed the importance of lens design fundamentals as a complement to computer-aided design.”

“At that time, ray tracing speed was a major bottleneck in the lens design process. Now that lens designers can trace rays in fractions of a second and have access to powerful computational tools like global optimization and AI are these same fundamentals needed?”

Bentley expressed one of Kingslake’s concerns about the advance of computing power: “Should we keep teaching the fundamentals?” she said. “One of his biggest fears was that we would forget what he called ‘our laboriously acquired knowledge of geometrical optics’ and substitute for it the mathematical problem of optimizing a merit function?”

“There is no question that computers have done wonders for lens design and have enabled far more advanced designs than thought possible. The issue at hand is if mastery of both lens design fundamentals and computer software is required for success,” she said.

“Unfortunately, the current educational landscape places much more emphasis on the latter than the former, and many of the fundamentals impressed by Kingslake have been lost, Bentley said.

However, three boxes of index cards belonging to Rudolf Kingslake were recently uncovered. Included in the collection are 171 lens design exam problems which present a fascinating perspective on lens design as it was taught in the pre-computer age. Bentley’s talk examined several of these forgotten problems and discussed how their solutions are still relevant for modern lens design today.

One of Bentley’s surprising conclusions from sharing some of the cards with her students was not so much that traditional optical teaching ideas might be lost to modern computing methods but that today’s optics learners often had a problem understanding cursive handwriting – never mind fundamental ray tracing concepts.

Mirror segments for the Extremely Large Telescope

Tuesday’s second plenary talk in the SPIE Optical Systems Design series was given by Camille Frapolli of Safran Reosc, France, and entitled Manufacturing ELT M1 segments: large optics in a smart factory.

In 2017, the European Southern Observatory (ESO) awarded a contract for the Polishing, integration and final figuring of the Segment Assemblies of the primary mirror (M1) for the Extremely Large Telescope (ELT) to Safran Reosc. Since then, the design and commissioning of a production unit dedicated to ELT M1 has been accomplished and the plant has been producing many mirrors since spring 2022.

In his talk, Frapolli introduced Safran Reosc’s smart factory, its processes and their automation that enabled reaching the current throughput of one mirror per day. He then presented the status of the project, some lessons learned and highlighted the successes that have been achieved so far.

With its “smart factory” approach, Safran Reosc has achieved a production rate in excess of four segments per week, sometimes as many as eight or 10 in a good week, a remarkable achievement for the series production of incredibly high-accuracy optics, said Frapolli.

The construction of ESO’s ELT has required the close involvement of multiple companies in Europe and Chile with ESO’s teams, highlighting how the telescope is a true international endeavour. The mirror segments were cast by glass maker Schott at its facility in Mainz, Germany, before being delivered to Safran Reosc in France for polishing.

After a PhD in physics at the Ecole Normale Supérieur, Camille Frapolli joined Safran REOSC in 2017 as a metrology engineer. He worked on the development of several ELT test benches until 2020, when he became the chief engineer on ELT M1. Since 2023, he has also been leading the optical design and new test bench development team at Safran REOSC.